1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display using a thin film transistor (hereinafter, referred to as a TFT) and in particular, to a drive control apparatus of the active matrix type liquid crystal display.
2. Description of the Related Art
The currently wide-spread TFT is an n-channel MIS (metal insulator semiconductor) transistor constituting of amorphous silicon and poly silicon. In a liquid crystal, in order to prevent deterioration due to a DC component, a positive polarity voltage Vd (0 to Vdmax) and a negative polarity voltage xe2x88x92Vd(0 to Vdmax) are alternately applied. The TFT turns on if a gate voltage Vg greater than a threshold value is applied and the TFT turns off if a gate voltage smaller than the threshold value is applied.
Recently, the active matrix type liquid crystal display is expected to have a multi sync and smaller dots for monitor. Here, the multi sync is to display a video signal having different drive frequencies or resolutions. However, when multi sync and smaller dots are obtained, a horizontal sync frequency becomes higher, which in turn shortens the gate on time of the TFT for the liquid crystal drive. This reduces the time for writing a video signal into the liquid crystal. Accordingly, when writing a positive polarity voltage near the gate on voltage, the writing cannot be completed in time. As a result, a DC component is applied to the negative polarity side, deteriorating the liquid crystal.
In order to solve this problem, Japanese Patent Publication 5-108032 discloses a liquid crystal drive method in which the positive polarity voltage and the negative polarity voltage of a video signal to be applied to the liquid crystal are not set evenly for a common voltage but the positive polarity voltage is increased according to one horizontal period for transferring a display data of a horizontal line. Thus, when a frame frequency is made faster or when a selection period of a horizontal line is reduced because of the increased number of display lines, it is possible to drive without applying a DC component to the liquid crystal.
However, a complicated configuration is required to increase the positive polarity voltage while leaving the negative voltage as it is. It is difficult to realize such a configuration only by adding a certain circuit to an existing source driver. The source driver is a circuit for applying a positive polarity voltage and a negative polarity voltage constituting a video signal to a drain of the TFT.
It is therefore an object of the present invention to provide an active matrix type liquid crystal display drive control apparatus which can prevent, with a simple configuration, generation of a DC component of a voltage applied to a liquid crystal when the horizontal sync frequency is increased.
The drive control apparatus according to the present invention is used for an active matrix type liquid crystal display comprising: a first substrate having display pixel electrodes and thin film transistors (TFT), each arranged at one of orthogonal intersections of a plurality of gate lines and a plurality of drain lines, wherein sources of the TFT are connected to the display pixel electrodes, and a second substrate having a common electrode and arranged so as to oppose to the first substrate via a liquid crystal layer. The drive control apparatus according to the present invention comprises: a frequency detection circuit for detecting a horizontal sync frequency, a gate-on voltage changing circuit for hanging a gate-on voltage of the TFT according to the horizontal sync frequency detected by the frequency detection circuit, and a common voltage changing circuit for changing a common voltage of the common electrode according to the horizontal sync frequency detected by the frequency detection circuit.
Hereinafter, explanation will be given on the function of the present invention in case that the TFT is an n-channel transistor.
If the horizontal sync frequency is low, it is possible to assure a sufficient gate-on time for the TFT. However, if the horizontal sync frequency becomes higher and the TFT gate-on time becomes shorter, the voltage applied to the liquid crystal is insufficient when a positive polarity voltage is applied to the TFT drain. As a result, a DC component is applied to the liquid crystal.
To cope with this, in the present invention, the TFT gate-on voltage is changed according to the horizontal sync frequency. That is, as the horizontal sync frequency is increased, the gate-on voltage is increased. In other words, as the horizontal sync frequency is lowered, the gate-on voltage is lowered. This increases the drain current and it is possible to obtain a sufficient voltage for application to the liquid crystal even when a positive polarity voltage is applied to the TFT drain.
However, as the gate-on voltage is increased, the feed through voltage is also increased proportionally. The feed through is a phenomenon that the voltage applied to the liquid crystal is shifted to the negative polarity voltage when the TFT is turned off from the on state. If the feed through voltage becomes great, there arises a problem that a remarkable flicker occurs.
To cope with this, in this invention, the common voltage of the liquid crystal is changed according to the horizontal sync frequency. That is, as the horizontal sync frequency is increased, the common voltage is decreased. In other words, as the horizontal sync frequency becomes lower, the common voltage is made higher. Here, the gate-on voltage when a positive polarity voltage is applied to the drain of the TFT may be made higher than when a negative polarity voltage is applied to the drain and the difference between them may be increased as the horizontal sync frequency is increased. Since the voltage applied to the liquid crystal is a difference between the source voltage and the common voltage, lowering of the common voltage increases the positive polarity voltage applied to the liquid crystal and lowers the negative polarity voltage (absolute value) applied to the liquid crystal. This eliminates the affect of the feed through voltage generated when the gate-on voltage is increased and accordingly, eliminates a DC component of the voltage applied to the liquid crystal.
Here, the configuration for changing the TFT gate-on voltage according to the horizontal sync frequency only changes the voltage supplied to a gate driver and can be realized only by adding a simple circuit to the existing gate driver. The gate driver is a circuit for applying gate-on voltage as a scan signal to the TFT on horizontal line basis. Moreover, the configuration for changing the common voltage according to the horizontal sync frequency only changes a voltage supplied to the existing common electrode of the liquid crystal and can be realized only by adding a simple circuit.
Moreover, if the TFT is a p-channel transistor, it turns on when a negative polarity voltage is applied to the gate. Accordingly, as the horizontal sync frequency becomes higher, the gate-on voltage is lowered and the common voltage is increased. In other words, as the horizontal sync frequency is lowered, the gate-on voltage is increased and the common voltage is lowered. Here, the gate-on voltage when a negative polarity voltage is applied to the drain of the TFT may be lowered than the gate-on voltage when a positive polarity voltage is applied to the drain and the difference between them may be increased as the horizontal sync frequency is increased.